Think of the eye as a camera: light enters, is focused by lenses, forms an image on a screen (retina), and the brain interprets signals.

• ★Cornea: Transparent front surface. It does about two-thirds of the focusing because it has a curved surface and a different optical density from air. (Analogy: the camera’s front glass.)
• ★Iris: The coloured ring that adjusts how much light gets in by changing the pupil size — like the aperture on a camera.
• ★Pupil: The black circular opening; it gets smaller in bright light and larger in dim light.
• ★Crystalline lens: A flexible, transparent lens behind the pupil. It fine-tunes focus for near and far objects by changing shape (accommodation). Imagine a zoom lens that changes curvature.
• ★Retina: A thin layer at the back with two main types of photoreceptors:
  • Rods — very sensitive; work in low light; detect black & white and shapes.
  • Cones — require brighter light; detect colour (three types: red, green, blue sensitive).
• ★Optic nerve: Carries electrical impulses from retina to brain. Where nerve fibers leave, there are no photoreceptors — this is the blind spot.
• ★Eyeball size: Average diameter ≈ 2.3 cm. Small changes in shape can change vision (myopia/hypermetropia).

Accommodation is the process by which the lens changes its curvature to focus on objects at different distances.

• ★Distant vision: Ciliary muscles relax → Suspensory ligaments pull the lens thin → Focal length increases → Image focuses on retina.
• ★Near vision: Ciliary muscles contract → Suspensory ligaments relax → Lens becomes thicker and more curved → Focal length decreases → Near objects focused.
• ★Example: Reading a book makes the lens thicker; looking at the sky makes it thinner.

A. Myopia (Short-sighted)

• ★What you see: Near objects clear; distant objects blurred.
• ★Why: Eye too long or lens too strong → image forms in front of retina.
• ★Correction: Concave (diverging) lens moves the image back on the retina. Prescription powers are negative (e.g., −1.5 D).
• ★Everyday check: If distant road signs are fuzzy, suspect myopia.

B. Hypermetropia (Far-sighted)

• ★What you see: Distant objects clear; nearby objects blurred.
• ★Why: Eye too short or lens too weak → image forms behind the retina.
• ★Correction: Convex (converging) lens brings the focus forward onto the retina (positive power, e.g., +2.0 D).

C. Presbyopia — age-related

• ★With age, the lens becomes less elastic and ciliary muscles weaken → less accommodation.
• ★Near point moves away; common after 40 years.
• ★Corrected with reading glasses (convex) or bifocals (two powers in one lens).

• ★A prism is a triangular wedge of glass. When light enters, it bends because glass is denser than air (refracts).
• ★Angle of prism (A): the apex angle between the two refracting faces.
• ★Angle of deviation (D): total change in direction of a ray after passing the prism.

• ★White light contains many wavelengths (colours). In glass, different wavelengths travel at slightly different speeds → different bending angles.
• ★Order of colours (from red to violet): V I B G Y O R (Violet bends the most).

Newton’s Prism Trick

• ★After the first prism creates a spectrum, a second inverted prism recombines the colours back to white — proving colours are components of white light, not created by the prism.

Rainbow Formation (simple steps)

1. Sunlight enters a water droplet — refraction splits colours.
2. Light reflects inside the droplet.
3. Light refracts again when leaving — observer sees a coloured arc opposite the Sun.

Twinkling of Stars

• ★Starlight passes through many layers of moving air with different temperatures and densities. These random refractive changes make the apparent brightness and position vary rapidly → twinkle.

Planets vs Stars

• ★Planets appear as tiny discs (not point sources), so the twinkling effects average out — planets usually do not twinkle.

Sunrise & Sunset

• ★Atmospheric refraction bends sunlight, so the Sun is visible slightly before it geometrically rises and after it sets — roughly about 2 minutes each side (varies by location & conditions).
• ★The Sun looks a bit flattened near the horizon due to differential refraction of its top and bottom edges.

• ★Tyndall effect: Tiny particles scatter light so the beam becomes visible (e.g., light through fog or dust).
• ★Rayleigh scattering (simple): Air molecules are much smaller than visible wavelengths; they scatter shorter wavelengths (blue/violet) more strongly than longer ones (red).

Why is the sky blue (in simple words)?

Sunlight hits air molecules; blue light is scattered in all directions and reaches our eyes from everywhere above, making the sky appear blue.

Why are sunsets red?

Sunlight passes through more atmosphere near the horizon. Most blue light is scattered out, leaving the longer red wavelengths to reach your eyes.

Danger signals and red colour

• ★Red travels farthest through fog and smoke (least scattering) — that’s why warning lights and tail lights are red.

• ★Eyes should ideally be removed within 4–6 hours after death for the corneas to be usable.
• ★One donor may restore sight to up to 4 people through corneal transplants.
• ★Most people can donate (including those with diabetes or past cataract surgery). Certain infectious diseases disqualify donors.

• ★Eye is like a camera: Cornea + lens focus light onto retina; optic nerve → brain.
• ★Accommodation: Lens changes shape (ciliary muscles) for near/far focus.
• ★Myopia: image in front of retina — concave lens.
• ★Hypermetropia: image behind retina — convex lens.
• ★Dispersion: prism separates white light into colours — rainbow is natural dispersion.
• ★Scattering: Rayleigh scattering → blue sky; sunsets red.